This invention relates to a heated type air flow rate sensor for measuring an air flow rate by using a heating resistor formed in a diaphragm of which the circumference is fixed, and more particularly, a heated type air flow sensor having a structure suitable for measuring the intake pipe pressure of an automobile internal combustion engine.
A prior art heated resistor type sensor utilizes a structure as shown in Japanese Patent Prepublication No. 502966/1991. In this structure, a silicon rim to which p-type doping has been effected is positioned between a silicon substrate having the (100) crystal orientation and a thin dielectric diaphragm and under the highest point of a sandwich structure and a sensing element, and the silicon substrate has a taper formation thereon facing the silicon rim.
In the heated type air flow rate sensor, dust contained in the air, which is the object of measurement, can often destroy the diaphragm when it collides against the diaphragm, whereby it is often the case that the characteristic of the heated type air flow rate sensor deteriorates. The object of this invention is to solve this problem.
Moreover, if an automobile engine controlling apparatus employing the heated type air flow rate sensor of which the characteristic has thus deteriorated is used, its control accuracy decreases, whereby it is often the case that the amount of emission of air pollutant and the amount of consumption of fuel are increased. The object of this invention is to resolve this problem.
An object of this invention is to provide a heated type air flow rate sensor which prevents it from deterioration of the characteristics due to dust contained in the air, and thereby limits the emission of air pollutants and the amount of consumption of fuel to a small amount.
By forming two or more tapers on a diaphragm fixing flange and making the angle of the furthest inside taper smaller than the angle of a taper on the outside thereof, the destruction of the diaphragm is avoided.
Also, by using Si monocrystal and a Si3N4 film as materials that make up a fixing portion and the diaphragm, respectively, and by making the angle of the first stage taper below 15 degrees, preferably below 5 degrees, and having the thickness of its root above 12 xcexcm, the destruction of the diaphragm is avoided.
Also, in a method of forming a heated type air flow rate sensor in which a mask with an opening is formed on the backside of the Si monocrystal substitute having a thin film formed thereabove, the substrate is partially removed by carrying out etching and the thin film diaphragm and a fixing portion for supporting this are formed, etching is carried out using a mask having an opening which is smaller than a final substrate removed portion. Thereafter, the diaphragm and the diaphragm fixing portion are formed by carrying out the etching with the mask opening being expanded, whereby the destruction of the diaphragm is avoided.
Also, by forming an inner taper portion with a resin, a more flexible structure is provided, whereby the destruction of the diaphragm is avoided.
Also, by providing an automobile internal combustion engine controlling apparatus utilizing the heated type air flow rate sensor, using any one of the above-mentioned means, the characteristic deterioration of the air flow meter is prevented, control precision is improved, and the above-mentioned objects are achieved.